Long life lubricating oil with enhanced oxidation and nitration resistance

ABSTRACT

A long life lubricating oil, as evidenced by a reduction in oil thickening, oxidation, nitration and low deposit-forming tendency, comprises a major amount of a base oil of lubricating viscosity and a minor amount of a mixture of neutral and overbased metallic detergents and at least one trinuclear molybdenum compound.

This application is a Continuation-in-Part of Non-Provisional application Ser. No. 10/683,651 filed Oct. 10, 2003 which in turn claims the benefit of U.S. Provisional Application No. 60/419,745 filed Oct. 18, 2003.

FIELD OF INVENTION

The present invention relates to lubricating oils, especially gas engine oils, having improved deposit control and extended life as evidenced by a reduction in viscosity increase, oxidation, nitration and deposit formation.

BACKGROUND OF INVENTION

Gas fired engines are typically used in the oil and gas industry to drive compressors that compress natural gas at well heads and along pipelines. These engines are large, having up to 16 cylinders and often generate between 500 to 3000 HP. The nature of their use requires them to be able to run continuously near full load conditions, shutfing down only for maintenance such as for oil changes. Under these operating conditions severe demands are placed on the engine lubricant. Indeed, because the lubricant is subjected to a high temperature environment, the life of the lubricant often is limited by oil oxidation processes. Additionally, natural gas fired engines generate nitrogen oxides (NO_(X)) that can limit lubricant life by oil nitration processes. Therefore, gas engine operators are constantly seeking gas engine oils that have improved resistance to oxidation and nitration. The need to maintain clean engine components has grown more challenging, with longer drain intervals and generally more stress on the crankcase oil. Control of deposits on pistons, piston rings and intake and exhaust valves is particularly important.

In addition to controlling oxidation and nitration properties of a gas engine oil it also is necessary to control the ash content of the oil because the lubricant ash acts as a solid lubricant protecting the valve/seat interface of the engine. For this reason gas engine oils are classified according to their ash content. The classifications are: Ash Designation Ash Level, wt % (ASTM D874) Ashless Ash < 0.1% Low Ash 0.1% < Ash < 0.6% Medium Ash 0.6% < Ash < 1.5% High Ash Ash > 1.5%

The ash level of the lubricant often is determined by its formulation components, with metal-containing detergents and metallic-containing antiwear additives contributing to the ash level of the lubricant. Gas engine manufacturers specify the appropriate lubricant ash level for correct operation of a given engine. Thus, manufacturers of 2-cycle engines often specify use of an ashless oil. Manufacturers of 4-cycle engine may specify low, medium or high ash depending upon the level required for engine cleanliness and durability.

As is known in the art, additives are used in lubricants to perform numerous functions. For example, some are antioxidants, some are friction modifiers; and some are extreme pressure agents. Indeed some additives perform more than one function. Also as is known in the art, additives will lose their effectiveness if they are improperly combined. Therefore, extreme care must be exercised in combining various additives to assure both compatibility and effectiveness. For example, some friction modifiers affect metal surfaces differently than do antiwear agents. When both are present, friction-reducing and antiwear additives may compete for the surface of the metal parts which are subject to lubrication. This competition can produce a lubricant that is less effective than is suggested by the individual properties of the additive components.

Accordingly, the components of a gas engine lubricant need to be selected to meet the specified ash level and to provide, among other functions, a high level of oxidation and nitration resistance. Whether selected components and their amounts can be balanced to meet desired specification is not a priori predictable.

SUMMARY OF INVENTION

Simply stated the present invention relates to a lubricating oil, especially useful as a gas engine oil, comprising a major amount of a base oil of lubricating viscosity; effective amounts of a mixture of neutral and overbased metallic detergents; and, a minor amount of at least one trinuclear molybdenum compound.

The lubricant composition of the invention provides enhanced deposit control and has extended life as evidenced by reductions in oxidation and nitration relative to commercial and reference oils, and may also be compatible with other standard additives used in formulating commercial lubricating compositions.

DETAILED DESCRIPTION OF INVENTION

The lubricant compositions of the present invention include a major amount of a base oil of lubricating viscosity. Suitable base oils include natural and synthetic oils and mixtures thereof in API Categories I, II and III, and having a kinematic viscosity in the range of about 9 to about 16 cSt at 100° C. and preferably from about 9 to 13 cSt at 100° C.

The metallic detergent contained in the composition of the invention is a mixture of neutral and overbased metal sulfonates, phenates and alkylsalicylates. The metals may be alkali and alkaline earth metals and preferably are alkaline earth metals, especially calcium and barium. Examples of suitable neutral metallic detergents are calcium sulfonates and calcium alkylsalicylates having a TBN of from 10 to 100. Examples of higher TBN or overbased metallic detergents are calcium phenates, sulphonates and alkylsalicylates having a TBN of about 110 to 400. The amount of the neutral and overbased metallic detergent is chosen having regard to the desired TBN of the final product and especially having regard to the desired sulfated ash of the final product. Preferably the mixture of neutral and overbased metallic detergents is sufficient to provide the composition with a sulfated ash in the range of about 0.2 mass % to about 2.0 mass %.

The compositions of the present invention also include a minor amount of at least one trinuclear molybdenum compound. A preferred trinuclear molybdenum compound is represented by the formula Mo₃S_(k)L_(n)Q_(z) where L represents independently selected ligands, n varies from 1 to 4, k varies from 4 to 7 and Q is selected from the group consisting of neutral electron donating compounds including water, amines, alcohols, phosphines and ethers and z ranges from 0 to 5. Such compounds and their method of preparation are disclosed in great detail in U.S. Pat. No. 6,232,276 B1 which is incorporated herein by reference. In the present invention L preferably is dithiocarbamate, n is 4, k is 7, Q is an electron-donating compound, and z is 0. The trinuclear molybdenum compound preferably constitutes from 0.1 vol % to about 2.0 vol % based on the total volume of the total lubricant composition.

Optionally, the composition may contain an ashless dihydrocarbyl thiocarbamoyl in combination with the trinuclear molybdenum compound.

Suitable dihydrocarbylthiocarbamoyl compounds are represented by the formula

where R₁, R₂, R₃ and R₄ are the same or different and each represents an alkyl group of 3 to 30 carbon atoms, X represents S, S—S, SCH₂_(y)S, S—CH₂CH₂(CH₃)—S and y is an integer of 1 to 3.

Preferably the combined materials i.e., the trinuclear molybdenum compound and dihydrocarbyl thiocarbamoyl compounds will constitute from about 0.1 vol % to about 2.0 vol % based on the volume of the total lubricant composition; however, the amount of dihydrocarbyl thiocarbamoyl will not exceed about 1.90 vol %.

The fully formulated oil may contain additional, typical additives known to those skilled in the industry, used on an as-received basis.

Thus, the fully formulated oil may contain dispersants of the type generally represented by succimides (e.g., polyisobutylene succinic acid/anhydride (PIBSA)-polyamine having a PIB moiety molecular weight of about 700 to 2500). The dispersants may be borated or non-borated. The dispersant can be present in the amount of about 0.5 to 8 vol %, more preferably in the amount of about 1 to 6 vol %, most preferably in the amount of about 2 to 4 vol %.

Antioxidants may be of the phenol (e.g., o,o′ditertiary alkyl phenol such as ditertbutyl phenol), or amine (e.g., dialkyl diphenyl amine such as dibutyl, octyl buty, or dioctyl diphenyl amine) type, or mixtures thereof. More preferably, the antioxidants will be hindered phenols, or aryl amines which may or may not be sulfurized. Antioxidants can be present in the amount of about 0.05 to 2.0 vol %, more preferably in the amount of about 0.1 to 1.75 vol %, most preferably in the amount of about 0.5 to 1.5 vol %.

Metal deactivators may be of the aryl thiazines, triazoles, or alkyl substituted dimercapto thiadiazoles (DMTD's), or mixtures thereof. Metal deactivators can be present in the amount of about 0.01 to 0.2 vol %, more preferably in the amount of about 0.02 to 0.15 vol %, most preferably in the amount of about 0.05 to 0.1 vol %.

Antiwear additives such as metal dithiophosphates (e.g., zinc dialkyl dithiophosphate, ZDDP), metal dithiocarbamates, metal xanthates or tricreeylphosphates may be included. Antiwear additives can be present in the amount of about 0.05 to 1.5 vol %, more preferably in the amount of about 0.1 to 1.0 vol %, most preferably in the amount of about 0.2 to 0.5 vol %.

Pour point depressants such as poly(meth)acrylates, or alkyl-aromatic polymers may be included. Pour point depressants can be present in the amount of about 0.05 to 0.6 vol %, more preferably in the amount of about 0.1 to 0.4 vol %, most preferably in the amount of about 0.2 to 0.3 vol %.

Antifoamants such as silicone antifoaming agents can be present in the amount of about 0.001 to 0.2 vol %, more preferably in the amount of about 0.005 to 0.15 vol %, most preferably in the amount of about 0.01 to 0.1 vol %.

Viscosity index Improvers (VII's) may be any polymer which imparts multifunctional viscosity properties to the finished oil, including materials such as olefin copolymers, polymethacrylates, styrene diene block copolymers, and star copolymers the VII's may also be multifunctional from the perspective of offering secondary lubricant performance features such as additional dispersancy. VII's can be present in the amount of up to 15 vol %, more preferably in the amount of up to 13 vol %, most preferably in the amount of up to 10 vol %.

Lubricating oil additives are described generally in “Lubricants and Related Products” by Dieter Klamanm, Verlag Chemie, Deerfield, Fla., 1984, and also in “Lubricant Additives” by C. V. Smalheer and R. Kennedy Smith, 1967, pages 1-11, the disclosures of which are incorporated herein by reference.

The present invention is further described in the following non-limiting examples and comparative examples.

EXPERIMENTAL

A. Lab Nitration Screener Test

A lab nitration screener test was used to assess the oil life performance of various oil compositions. The test results identify a number of parameters including oil viscosity increase, oxidation, and nitration. All measurements are reported on a relative basis (unless otherwise indicated) so that results greater than unity represent greater levels of degradation. Numerically lower relative results represent a measure of longer oil life. In each test, a Reference Oil is tested and results are reported as a ratio of the result for the test oil divided by the result for the Reference Oil. Thus, if a tested oil has an oxidation result of 1.0, then it has oxidation performance equal to that of the Reference Oil. If the tested oil has an oxidation result less than 1.0, then the tested oil demonstrates oxidation performance superior to that of the Reference Oil

B. Lab Deposit Screener Test

An in-house proprietary deposit screener test was used to assess the deposit tendency of crankcase oils. The test reasonably mimics the engine conditions likely to cause deposits on valves and in the piston-ring zone. Commercial oils of known performance in severe field situations were used to develop the test and provide reference oils. The final conditions and protocol achieved excellent oil performance discrimination for the known commercial oils. The test measures the weight of lubricant-derived deposits that accumulate on a weighed aluminum coupon, under specific conditions of temperature, test length and metal surface-oil contact.

EXAMPLES AND COMPARATIVE EXAMPLES

Tables 1 and 2 below detail series of formulations which demonstrate the invention. In the Examples in Table 1, the base oil in all cases was a heavy API Group II basestock. Comparative Oil 1, Reference Oil 1 and Examples 1 and 2 contained the same viscosity index improver (VI). Comparative Oil 2 is a current commercial oil based solely on API Group II basestocks. Reference Oil 1 and Example Oils 1 and 2 each contained the same mixture of neutral and overbased metallic detergents, ashless dispersant, ZDDP, pour point depressant, metal passivator, VI improver, and antifoamant. Reference Oil 1, however, employed a phenolic antioxidant whereas the oils of Examples 1 and 2 employed a trinuclear molybdenum compound, Mo₃S₇(DTC)₄, and a mixture of a trinuclear molybdenum compound, Mo₃S₇(DTC)₄, and an ashless dihydrocarbyl thiocarbamoyl, (S₂CNR₂)₂CH₂, respectively. Comparative Oil 1 used a commercially available gas engine oil additive package, Oloa 1255 sold by Chevron Chemical Company. Oloa 1255 is one of the most widely sold gas engine oil packages and therefore represents a “benchmark standard” against which other engine oil formulations may be measured.

The results in Table 1 show that the oils of the present invention, Examples 1 and 2, provided performance superior to that of both of the Comparative oils and the Reference Oil, in terms of reduced oxidation, nitration and viscosity increase. The invention examples provided superior oil life despite the absence of conventional aminic and phenolic antioxidants. The small negative normalized viscosity increase values for the invention examples simply reflect that there was no significant change in viscosity, unlike the Comparative and Reference oils. TABLE 1 Comparative Reference Comparative Oil 1 Oil 1 Example 1 Example 2 Oil 2 Component (vol %) Group II basestock 87.90  90.00  90.00  90.00  — NGEO commercial 9.6  — — — — additive package Phenolic antioxidant 1.00 1.00 — Moly trimer — — 1.00 0.50 — Ashless dihydrocarbyl — — — 0.50 — thiocarbamoyl Balance of additives 1.50 9.00 9.00 9.00 — Commercial Oil — — — — 100.00  Properties KV, cSt @ 100° C. 13.25  13.14  13.20  13.13  13.51  Test Results Nitration Screener Test Oxidation (relative) 1.76 1.00 0.86 0.79 1.57 Nitration (relative) 1.55 1.00 1.15 0.91 1.48 Viscosity Increase 1.70 1.00 −0.16   −0.12   0.73 (relative)

In the Examples shown in Table 2 the low ash formulations used an API Group II basestock, whereas the medium ash formulations used a Group I basestock (except for Comparative Oil 4). Reference Oil 2 is a commercial premium low ash gas engine oil that uses API Group II basestocks and Reference Oil 3 is a commercial premium medium ash gas engine oil that uses API Group I basestocks. Comparative Oils 3 and 4 are additional low ash and medium ash gas engine oils, respectively, that are believed to employ commercially available additive packages. Comparative Oils 3 and 4 both use API Group II basestocks.

Reference Oils 2 and 3, and Examples 3, 4, 5 and 6 mostly use similar mixtures of neutral and overbased metallic detergents, ashless dispersant, ZDDP, pour point depressant, metal passivator, VI improver, and antifoamant. A hindered phenolic ashless antioxidant was used in all 4 examples of Table 2, at different treats for the low ash and medium ash formulations; the moly trimer was Mo₃S₇(DTC)₄].

Each of the formulations was subjected to the laboratory deposit screener test which measures the weight of lubricant derived deposits that accumulate on a weighed metal panel. The Invention Examples 3 and 4 contained 0.25 and 0.1 vol % of the moly trimer, respectively; these inventions gave unusually low deposit weights (12.0 and 18.4 mg) and much less deposit coverage of the coupon surface than the low ash Reference and Comparative oils. The lower deposit weights of the medium ash oils in general is a reflection of the higher detergent treats (ash) for medium ash oils. Nevertheless, use of the moly trimer at 0.5 and 0.1 vol % treats resulted in outstanding deposit control of 2.2 and 1.0 mg for Examples 5 and 6, respectively. In these 2 cases, panel surface coverage by deposits was only a small fraction of total coupon area. The Invention Examples 3, 4, 5 and 6 provide exceptional deposit control, beyond that of current commercial technology and beyond what could be expected for the detergent treat rate. TABLE 2 Low Ash Medium Ash Formulation Description Reference Comparative Invention Invention Reference Comparative Invention Invention Oil 2 Oil 3 Example 3 Example 4 Oil 3 Oil 4 Example 5 Example 6 Component Basestock Description (vol %) Group II Group II Group II Group II Group I Group II Group I Group I Commercial sample 100.00 — — — — — — — Commercial sample — 100.00 — — — — — — Commercial sample — — — — 100.00 — — — Commercial sample — — — — — 100.00 — — Group II Basestock — — 90.65  90.80  — — — — Group I Basestock — — — — — — 92.100 92.500 Balance of low ash additive — — 7.50 7.50 — — — — system Balance of medium ash — — — — — — 6.700 6.700 additive system Ashless antioxidant — — 1.50 1.50 — — 0.500 0.500 Proprietary molybdenum — — 0.25 0.10 — — 0.500 0.100 dithiocarbamate trimer PPD — — 0.10 0.10 — — 0.200 0.200 Kinematic Measured kV @ 100° C. 13.2 13.4 ˜13      ˜13      13.2 13.7 ˜13 ˜13 Viscosity, cSt Deposit Deposit Weight, mg 47.6 35.5 12.0  18.4  16.5 28.6 2.2 1.0 Screener Test 

1. A lubricating composition comprising: a major amount of an oil of lubricating viscosity; a mixture of neutral and overbased metallic detergents; and a minor amount of at least one trinuclear molybdenum compound.
 2. The composition of claim 1 wherein the trinuclear molybdenum compound is represented by the formula MO₃S_(k)L_(n)Q_(z) where L is an independently selected ligand, n is from 1 to 4, k is from 4 to 7, Q is a neutral electron donating moiety and z is from 0 to
 5. 3. The composition of claim 2 wherein the neutral and overbased metallic detergents are selected from the group consisting of alkali and alkaline earth metal sulfonates, phenates, alkylsalicylates and mixtures thereof.
 4. The composition of claim 3 wherein the amount of detergents is sufficient to provide the composition with a sulfated ash content in the range of about 0.2 wt % to about 2.0 wt %.
 5. The composition of claim 4 wherein the trinuclear molybdenum compound constitutes from about 0.1 vol % to about 2.0 vol % based on the volume of the total lubricant composition.
 6. The composition of claim 5 including a minor amount of a dihydrocarbyl thiocarbamoyl compound.
 7. The composition of claim 6 wherein the dihydrocarbyl thiocarbamoyl compound has the formula

where R₁, R₂, R₃ and R₄ are the same or different alkyl groups of from 3 to 30 carbon atoms; X is S, S—S, SCH₂_(y)S, S—CH₂CH₂(CH₃)—S; and y is an integer of 1 to
 3. 8. A lubricating oil composition comprising: a major amount of an oil of lubricating viscosity; a mixture of neutral and overbased metallic detergents selected from the group consisting essentially of alkaline earth metal sulfonates, phenates, alkylsalicylates and mixtures thereof in an amount sufficient to provide the composition with a sulfated ash content of from about 0.2 wt % to about 2.0 wt %; a trinuclear molybdenum compound having the formula Mo₃S_(k)L_(n)Q_(z) where L is an independently selected ligand, n is from 1 to 4, k is from 4 to 7, Q is a neutral electron donating moiety, and z is from 0 to 5; from 0 to 2.0 vol % of a dihydrocarbyl thiocarbamoyl compound having the formula;

where R₁, R₂, R₃ and R₄ are the same or different alkyl groups of from 3 to 30 carbon atoms; X is S, S—S, SCH₂_(y)S, S—CH₂CH₂(CH₃)—S; and y is an integer of 1 to 3; the amount of trinuclear molybdenum compound and dihydrocarbyl thiocarbomyl compound being in the range of about 0.05 vol % to about 2.0 vol %.
 9. A method for enhancing the life of a lubricating oil as evidenced by a reduction in viscosity increase, oxidation and nitration, and for enhancing the oil's resistance to deposit formation, the method comprising: adding to the lubricating oil additives comprising: a mixture of neutral and overbased metallic detergents selected from the group consisting essentially of alkaline earth metal sulfonates, phenates, alkylsalicylates, and mixtures thereof in an amount sufficient to provide the composition with a sulfated ash content of from about 0.2 wt % to about 2.0 wt %; at least 0.1 vol % of a trinuclear molybdenum compound having the formula Mo₃S_(k)L_(n)Q_(z) Where L is an independently selected ligand, n is 1 to 4, k is 4 to 7, Q is a neutral electron donating moiety, and z is 0 to 5; a dihydrocarbylthiocarbomyl compound; the amount of trinuclear molybdenum compound and dihydrothiocarbomyl compound being in the range of 0.1 vol % about 2.0 vol %.
 10. The method of claim 9 wherein the dihydrothiocarbomyl compound has the formula

where R₁, R₂, R₃ and R₄ are the same or different alkyl groups of from 3 to 30 carbon atoms; X is S, S—S, SCH₂_(y)S, S—CH₂CH₂(CH₃)—S; and y is an integer of 1 to
 3. 